The present invention relates to a method for preparing an amorphous polymer chain in elastomers by a novel technique differing from conventional ones and to a method for preparing an elastomer having excellent mechanical strength as well as rubber elasticity. Generally, elastomers (rubber elastic bodies) are substances which can be obtained by chemically or physically bonding a part of linear polymeric substance (raw rubber: raw material rubber) whose molecules are active in rotational movement at room temperature. Typical elastomers include natural rubber, a part of which is crosslinked with sulfur, peroxides, or the like. A natural rubber derived elastomer is a polymeric compound comprising monomer components arranged with stereoregularity and is amorphous in a normal state but it is highly oriented to behave like a crystallized polymer when excessive deformation is applied thereto and, hence, is an ideal elastomer that has a sufficient resistance to wear and a sufficient strength.
According to the prior art, various elastomers with synthetic amorphous polymer chains have been designed or prepared by processes including the following:
First, there are cited homopolymers such as isoprene rubber (IR), butadiene rubber (BR), and chloroprene rubber (CR). The crystallizability of these polymers depend largely on their micro structures. Here, note that IR is a polymer of isoprene which is a polymerization unit of natural rubbers (NR) and designed to have high properties similar to natural rubbers by selectively increasing cis-1,4- bonds to the level observed in natural rubbers. BR is known to be a polymer whose crystallizability can be controlled by precisely controlling the micro structures such as cis-1,4-, trans-1,4-, -1,2- or -1,3-addition. Also, CR is a crystallizable polymer which comprises mainly a trans-1,4- structure.
Second, there are cited random copolymers such as styrene/butadiene rubber (SBR) and ethylene/propylene rubber (EPR), which are made amorphous by random copolymerization using as one of comonomers a monomer capable of forming a homopolymer having a low glass transition temperature (Tg), for example, butadiene, ethylene, etc.
Third, there are cited alternate copolymers such as tetrafluoroethylene/propylene rubber which are made amorphous by alternate copolymerization to vary the length of the unit structure constituting the polymer.
Fourth, there is cited graft or block copolymers such as block SBR, in which two units, i.e., a rubber component and a resin component, are arranged in the form of a main chain and a graft attached thereto or of blocks linked to each other to thereby render the resulting polymer amorphous.
Fifth, there are cited modified polymers such as chlorosulfonated polyethylene rubber (CSM) and chlorinated polyethylene (CM), in which the corresponding crystallizable homopolymer (e.g., homopolymer of ethylene) having a low Tg is modified to render the resulting polymer noncrystallized or amorphous.
Sixth, there are cited copolymers of a liquid or low crystallizability oligomer having reactive groups on the terminals of the molecule with a chain extender capable of reacting with the terminals. This approach is used typically in the preparation of millable polyurethanes.
As described above, in order to prepare acceptable amorphous polymer chains having vivid molecular motility, it has conventionally been considered necessary to reduce intermolecular force and minimize steric hindrance for the rotation of molecules and, hence, the above described amorphous polymer chains have been designed and prepared. Accordingly, conventional methods for designing amorphous polymer chains include: (1) arranging the double bonds in the polymer chain stereoregularly; (2) randomly or alternately copolymerizing a plurality of monomers having different properties such as structural unit length and showing crystallizability when converted into homopolymer to thereby decrease the crystallizability and render the polymer amorphous; (3) graft or block copolymerizing a plurality of monomers containing at least one monomer to be made amorphous to render the polymer amorphous as a whole; (4) making the polymer amorphous by chemically modifying a homopolymer which shows crystallizability, and; (5) polymerizing an amorphous oligomer.
In addition, there have been known amorphous polymer chains having two or more of the above described features in combination. For example, in the case of CR, in order to cope with the need for continued use at low temperatures, there have been used those polymers having a decreased crystallizability by copolymerization with one or more other monomers (cf. German Patent Publication DE-A-2,235,811).
Along with recent diversification of industry, a wide variety of elastomers having various functions have been demanded and conventional elastomers have become difficult to cope with such demand. More particularly, there has been awaited development of high performance elastomers having simultaneously those characteristics which have conventionally been considered contradictory to each other. To meet with such needs, the above described conventional approaches have been unsuccessful.
For example, suppose that it is intended to prepare polymer chains having reversal property that they crystallize when stretched while they become amorphous when relaxed. Then, it is necessary to prepare amorphous polymer chains which have structural regularity. However, it has been difficult to perform fine or precise control or adjustment of the polymer arrangement to the extent as desired by the above described conventional methods.
For example, it has been considered that such a high performance elastomer can be prepared using a catalyst such as metallocene or the like with simultaneously controlling stereoregularity, comonomer composition, and molecular weight distribution. However, the monomers which can be used are limited to olefins so that it is difficult to high performance elastomers having desired characteristics.